Fault alarm radio repeater system



March 3, 1959 Filed June 11, 1953 EAST TERMINAL REPEATER REPEATER wEsTTERMINAL FAULT ALARM RADIO REPEATER SYSTEM H. C. LlKEL FIG.5

4 Sheets-Sheet 1 l nu. vvv" Ann TO IF INVENTOR.

H. C. LIKEL m44 ff. WWW- ATTORNEY March 3, 1959 H. c. LlKEl. 2,876,341

FAULT ALARM RADIO REPEATER SYSTEM Filed June ll, 1953 4 Sheets-Sheet 2ENTRY BATTERY 24 24 24C D 53D y z lf 52 f? FIG.4

INVENTOR.

H. C. LIKEL.

l BYMM/w.

l ATTORNEY March 3, 1959 H. c. LIKEL FAULT ALARM RADIO REPAS-ATER SYSTEM4 Sheets-Sheet 3 Filed June l1, 1953 ATTORNEY March 3, 1959 H. c. LIKELFAULT ALARM RADIO REPEATER SYSTEM 4 Sheets-Sheet 4 Filed June 1l, 1955 n.DE 0..

ATTORNEY United States Patent 2,876,341 FAULT ALARM RADIO REPEATERSYSTEM Harry C. Lili-rel, Brooklyn, N. Y., assignor to The Western Thisinvention relates to fault alarm systems for microwave radio repeaters.

A typical radio system comprises two terminal stations and a pluralityof repeaters located at fixed distances between the terminals. Equipmentmalfunctions, the occurrence of fading and other troubles at therepeaters may cause interruption of transmission. Since the repeaterstations are customarily unattended, it is important that the terminalstations receive information concerning a faulted repeater in order thataction can be taken to clear the fault and restore service between theterminal stations.

It is, therefore, an object of this invention to provide means whereby aterminal station is informed of low input to any repeater in the system.

It is a further object o-f this invention to provide means whereby aterminal station will be informed of low output level of a repeater.

A further object of this invention is to provide means whereby localconditions at the repeater such as entry into the building, lowtemperature and other information is communicated to the terminalstations.

A still further object of this invention is to provide means at arepeater to reestablish the carrier in the event of failure of service.

These and other objects will become apparent from the followingdescription of the invention taken in conjunction with the accompanyingdrawings in which- Fig. 1 is a schematic representation of a radio relaysystem showing two terminal stations and two repeaters;

Figs. 2, 3 and 4 illustrate diagrammatically the receiving andtransmitting apparatus at a repeater station along with the circuitconnections of the fault alarm system;

Fig. 5 is a circuit diagram of a service channel amplier and controlcircuit for carrier reestablishment apparatus; and

Fig. 6 shows the method of arranging Figs. 2, 3 and 4.

The system disclosed in Fig. l represents a typical radio relay systemfor two-way transmission of radio signals between two terminal stations.Intermediate the terminal stations are a plurality of repeaters whosepurpose is to relay received signals from a preceding repeater orterminal station to the next repeater or terminal station. The repeaterstations may be of any number although two are shown in Fig. l.

The repeater shown in Figs. 2, 3 and 4 is representative of any one of aseries of such repeaters linking two terminal stations. For purposes ofdescription the left side of Figs. 3 and 4 will be considered west andthe right side east. Each repeater is designed to relay both west toeast and east to west signals through the employment of two similar setsof equipment facing in opposite directions. Considering the west to eastrepeating equipment, there is provided a receiving antenna 10 whichapplies the incoming wave, which may for example have a carrierfrequency of 4040 mc., to a mixer 11 to which is also supplied a 3970mc. signal from a local crystal controlled oscillator and frequencymultiplier 13. The mixer 11 produces a 70 mc. intermediate frequencywhich is fed to a 70 mc. intermediate frequency amplifier 12 providedwith automatic gain control. The output of this amplier isapplied toaconfiice verter 14 and mixed with'a 180 mc. output from another localoscillator 15. Here again, the signal wave is translated complete withall modulation, to `110 mc. by the selection of the difference frequencycomponents. This wave is amplified by amplifier 16 and applied alongwith some of the output of oscillator 13 to a klystron power amplifyingtube 17 which feeds transmitting antenna 18 with a carrier'having afrequency of 4080 mc.,

i. e., the 3970 mc. of the local oscillator 13 plus the 110 mc. signal.

It is seen from the above description that the carrier transmitted bythe repeater is the same carrier that is.

received thereby, though at a different frequency due to the frequencytranslation function effected in the re- Therefore, unlike prior artrepeaters, there is,v no demodulating of the input carrier withsubsequent4 peater.

modulating of a new output carrier. Hence the neces-` sity for afrequency source of the fundamental carrier frequency output at therepeater is eliminated.

The elements that make up the receiving and transmitting apparatus ofthe repeater may be of any convenient design, and are here representedin block form since the details of these elements form no part of thepresent invention. In fact, other transmitting and receivingarrangements could be equally well employed with the fault alarm systemhereinafter described. It is of course understood that the frequenciesset forth here are by way of example only, and any other convenientfrequencies may be employed. v

It may be readily understood, that due to a malfunction of a precedingrepeater, or other cause, the input signal might be weak beyond theability of the repeater to restore it to its normal value, or it mightbe absent completely. This condition must be made known to the operatorsat the terminal stations so that remedial steps may be taken.Accordingly, a sensitive relay 19l is coupled to the automatic gaincontrol circuit of the intermediate frequency amplifier 12. A SensitrolRelay, of the type shown in U. S. Patent No. 2,062,915 has been used forthis purpose and has been found to be very satisfactory because of itssensitivity and simplicity. The contacts of this relay are set to beheld open by a `predetermined value of current in the automatic gaincontrol circuit of the intermediate frequency amplifier 12. If thecurrent falls below this minimum value for which the relay is set, thecontacts will close and lock by virtue of the action of a smallpermanent magnet associated with them. Sensitrol relay 19 is arranged tocontrol a relay 21 so that upon closing of the contacts of the Sensitrolrelay, relay 21 is energized to close its four normally open contacts21A, 21B, 21C, and 21D. Contacts 21A complete a circuit'from a locallyavailable 110 volt 60 cycle source S over line 27 through normallyclosed relay contacts 22B, 23B, 24B and 25B to a fault tone audiooscillator 26. Contacts 21C, also connected to the source S, supply amodulating signal, over Wires 28 and 29, for the output of audiooscillator 26. Its modulated output is applied to both Ythe eastward andwestward transmitters over wires 31 and 32 which connect to localoscillator 15 to vary its frequency and consequently modulate thefrequency p of the carrier, thereby indicating to the terminal stafallsbelow a predetermined minimum for which the:

Patented Mar. 3, 1959 Sensitrol relay Ais set. Closure of these contactsserve to energize relay 36 thereby closing normally open contacts 36Aand 36B. Contacts 36A complete a. circuit from the 6.0 cycle lll) voltsource to the fault tone 'oscillator 26 over line `27 previously tracedin regard to the low input detecting relay 21.

It is seen, therefore, that in both a low input level and low outputlevel condition, the fault tone oscillator 26 is energized to frequencymodulate the carrier thereby indicating the faulted condition. However,only in the case of a low input level received by the repeater is thefault tone oscillator amplitude modulated by a 60 cycle modulatingfrequency (over wires 28 and 29). By this means a low input levelcondition is distinguished lfrom a low output condition.

As mentioned above, the Sensitrol relay vcontacts are brought togetherand held with the necessary pressure for good contact by the action of asmall permanent magnet. The 'strength lof this magnet is such that.should the `fade in signal pass 'and come back to normal, it wouldnevertheless not exert -'a force inthe Sensitrol relay great enoughtorelease the contacts. To overcome this, Ysolenoid release mechanisms areprovided which when pulsed force the contacts apart. Contacts 21B and36B of power relays 21 and 36 are connected in parallel so that theclosing of either, due to low input signal and low output signalrespectively, will complete a circuit from a source of llt) volt 60cycle power vover wire 37 to energize timer motor 3S of a release unit.The timer is a commercially available type set to rotate at A R. P.` M.Secured to the timer shaft is la cam 42 arranged to close normally opencontacts 43 to energize release solenoids 44 each of which is associatedwith a Sensitrol relay. The energizat'ion of a solenoid 44 will releasethe closed contacts of the Sensitrol relay associated therewith.

' It is seen, therefore, that upon detection of a low input signal orlow output signal, timer 38 is caused to energize to rotate cam 42.After an interval of four minutes cam actuated contacts 43 close toenergize solenoids -44 thereby releasing any Sensitrol relay that isclosed. When the Sensitrol contacts are opened, they will remain so ifthe faulted condition is cleared and current is back to the minimum forwhich they were set. If the fault is not cleared, the contacts willimmediately reclose and the cycle will be repeated. The time interval offour minutes was chosen as a compromise betweenthe desire to-stop thesignal as soon as possible and the need to .keep the wear-'and te'ar 'onthe 'Sensitrol relays down when 'an equipment difficulty causes thisaction to repeat for a considerable period of time.

A second cam 41 is secured on the timershaft to close normally opencontacts 45. This cam is positioned so that when cam 42 energizessolenoids 44 thereby releasing the Sensitrol relays, the motor circuitis not broken for several additional seconds. Otherwise the motorcircuit would break upon the closing of contacts 43 which wouldtherefore .remain closed, constantly energizing release solenoids 44thereby rendering theSensitrol relays unresponsive to a faultedcondition.

The fault tone keying unit 46 includes four relays 22, 23, 24 and 2Soperated by circuits (not shown) responsive to various local conditions.For example, relay 22 is energized upon unlawful entry into theapparatus building, relay 23 by low temperature, relay 24 by operationofthe equipment from batteries because of failure of the commercialpower supply and .relay 25 .by operation of the .equipment from agenerator for the sa'rne reason. Relays 22 to 25 each .have a set ofparalleled make contacts 22A to 25A. If any one .of these closes, lacircuit is completed from .a 110 volt 60 cycle source S over wire 4o7`toenergize a "one R. P. M. motor 49. .Energization of` anyoneof relays 22to 25 also rcloses one of a set of contacts 22C to 25Crespectively toclose a circuit. from the `110 volt`60 cycle-sources over wire S1 andthe-respective one of cam controlled switches52A .toSZDto fault 'i2,876,341 n l o tone oscillator 26. Switches 52A lto 52D are controlledby cams 53A to 53D respectively, secured to shaft 54 of motor 49. Asshown, each of these cams will close its associated switch 52A to 52D apredetermined number of times for one rotation of shaft 54. Cam 53A willclose switch 52A once, cam 53B will close switch 52B twice, etc.,thereby keying the oscillator 26 in accordance with the particular relay22 to 25 which is energized. The fault tone oscillator output istherefore coded and connects with the repeater transmitters over wires31 and 32 to frequency modulate the carrier thereby informing theterminal operators of the local faulted condition. It should be notedthat the groups of projections on cams 53A to .53D are spaced in time sothat the signals are easily distinguishable as a group and no confusionwill arise due to overlapping of signals should more than one localfault relay 22 to A125 become energized at the same time. Motor controlswitch is mechanically linked ,by insulating .bar Si) to cam controlledswitch 52A so that motor 49 is controlled by .cam 53A whereby after eachrevolution the motor may be stopped leaving cams 53A to 53D in theoriginal position if the local fault has cleared at that time. Thisprevents the motor -frorn stopping in the middle of a signal sequenceand causing confusion should the faulted condition clear at such a time.

The various fault detection circuits so far described maybe summarizedas follows:

(l) Low transmitter output at a particular repeater causes relay 36 toclose thereby energizing fault tone oscillator Z6. The latter willproduce a constant tone to frequency modulate the main carriers.

(2) Low input signal closes relay 21 to energize the fault toneoscillator 26 and also modulate the output therefrom with a cycle source(over Wire 29). The modulated output from the oscillator in turnmodulates the main carriers.

(3) Various local conditions .such as entry into the apparatus building,low temperature, etc., cause the fault tone oscillator to be energizedby coded pulses thereby modulating the main carrier in accordance withthe coded pulses.

Considering now aradio relay system comprising l0 repeaters between awest terminal and an east terminal numbered from west to east, and thewest to east circuit fails at repeater No. 3, there will be no west toeast carrier at .repeaters 4, 5, .6, 7, 8,9 or 1d. Thus, these stationscan not send any information to the east terminal, but since the systemherein disclosed sends notice of a fault in both directions, the westterminal will be informed that stations 3 to 10 are not operatingeastbound and therefore there is trouble eastbound at station No. 3-If,however, an additional failure develops anywhere in the westboundcircuit this information can not be made known to the terminal operatorsbecause neither carrier is received at the terminal stations. To avoidthis type of situation which would leave the terminal personnelcompletely in the dark as to Where the faults existed, it is necessaryto provide some means of reestablishing the carrier in the unfaultedpart of the system.

There is therefore provided a carrier reestablishinent oscillator 6) ofconventional construction and set to osciliate at'the intermediatefrequency which in the example herein -shown is mc. The output ofoscillator 60 is applied to the transmitter output over wire 59 therebyreestabli'shingl the carrier so that any fault signal applied to the`system will be transmitted to the .terminal station. The oscillator 6uis energized from a local source of power over a circuit includingcontacts 2lD of relay 21, line 58 and contacts 55A of relay S5. As setforth above, relay 21 is energized upon a low input signal received bythe repeater. Energization of this relay closes contacts 21D topartially complete the energizing circuit for oscillator 60. However,`the reestablishment oscillator is not energized 'at this time becausecontacts :55A are in their'normally open position. Relay 55 iscontrolled by a recognition circuit 56 which is responsive only to apredetermined signal, for example, a 2O kc. signal frequency modulatedat 60 cycles as derived from a service channel amplifier 57 at eachrepeater as a result of the reception of a kc. FM signal sent out at theterminal station. This signal is sent out only when it is desired tostart the carrier reestablishment oscillator. in Fig. 4, a failure ofthe eastward carrier will result in the closing of contacts 21D. Theoperator at the east terminal, who of course knows that the eastwardcarrier has failed, applies a 20 kc. FM signal to the west-l wardcarrier which is received by the westward receiver, amplified by audioamplifier 57 and applied to control circuit 56 to energize relay 55. Theenergizing circuit for the reestablishment oscillator 60 is thereforecompleted over the circuit from oscillator 69, closed contacts 55Aofrelay 55, wire 58, and contacts 21D to source S. The mc. output of theoscillator is applied to the eastward transmitter over wire 59 therebyreestablishing the carrier. The reestablishment oscillator remainsenergized only so long as the 2O kc. FM signal from the east terminal istransmitted and received at the repeater to maintain relay 55 energized.

Considering again the example of a failure of the eastward carrier atrepeater No. 3, the reestablishment oscillator at each of stations 3 to10 will be conditioned due to the failure of an input signal in theeastward direction. The east terminal operator will transmit a 20 kc.recognition signal which will initially energize all of the conditionedreestablishment oscillators. However, as stations No. 4 to No. 10receive the reestablished carrierfrom each preceding station, theirrespective reestablishment oscillators will deenergize, due to therelease of rclays 21 at each station, leaving only the oscillator atfaulted station No. 3 energized to reestablish the carrier. It is ofcourse unnecessary that the reestablishment oscillators at the unfaultedstations 4 to 10 remain energized.

Referring now to Fig. 5, there is shown a circuit diagram of the servicechannel amplifier 57 and control circuit 56.

The amplifier input signals, derived from the 70 mc. I. F

ampliers discriminator, are applied to tube 61 through a blockingcondenser 62. A load resistor 63 constitutes the input impedance of theamplifier. The output of tube 61 is capacity coupled by condenser 64 tothe control grid of tube 65. A parallel resonant circuit comprisingcondenser 66 and inductance 67 tuned to a frequency of 20 kc. isprovided in the plate circuit of tube 65. Across the parallel resonantcircuit is a condenser 68 and a variable resistor 69 the center tap ofwhich connects to the control grid of a third amplifying tube 71. Asecond parallel resonant circuit comprising condenser 72 and inductance73, tuned to a frequency of 20 kc. is connected in the plate circuit oftube 71. The response characteristic of the amplifier therefore has apeak in the neighborhood of 2O kc. in addition to voice frequencyresponse. Connected between the plate and screen grid of tube 71 is atransformer 74, the secondary winding of which is coupled to aloudspeaker 75 whereby a terminal operator can communicate with amaintainer at the repeater.

The amplifier is designed to handle frequencies in the ordinary speechrange. Speech signals will therefore be delivered to the loudspeaker 75,while the 20 kc. signals which are frequency modulated at a 60 cyclerate to actuate the recognition circuit sweep back and forth across theresponse peak of the amplifier and become amplitude modulated at 120cycles. This output is delivered to a crystal demodulator circuitcomprising rectifier 76, ca-

pacitor 77 and resistor 78. The rectified output of the demodulator isfed to the control grid of amplifier tube 79 by means of resistor 31 andcapacitor S2. A second crystal restifier 83 is connected to the outputof amplifier tube 79 whereby the signals are rectified, filtered by Jctmdenser 8,4 and applied to the grid of tube 85 in whose As shownAplate circuit relay 55 is connected. In the absence of the 20 kc.recognition signals, the plate current of tube 85 is insufficient toclose relay 55. However, upon receipt of the recognition signal, theplate current of this tube will increase to energize relay 55 therebyenergizing the carrier reestablishment oscillator 58 as previously setforth.

The above-described circuit provides various safeguards againstinadvertent operation of the recognition circuit by spurious signals.The response characteristic of the amplifier 57 is shaped to drop offrapidly above 3' kc. and have a high sharp peak near 20 kc. due to theparallel resonant circuit of condenser 72 and inductance 73, therebyproviding a high degree of selectivity for 20 kc. signals. Also, toeliminate any possibility of a strong low frequency signal on theservice channel getting to the grid of tube 79, the low frequencyresponse of the amplifier is purposely made poor. As a furtherprotection against spurious signals operating the circuit, the timeconstant of the rectifier and filter circuits are made such that asignal that swept through 2O kc. would not cause operation of the relay55 unless its recurrent rate was in the cycle range and persisted for atleast one second.

It is seen from the above, that the carrier reestablishment oscillatoris not put into operation automatically by the detection of low input oreven by failure of an input signal. This condition merely prepares theoscillator energization circuit, which is completed only upon receipt ofa recognition signal from a terminal station. The purpose of not havingthe reestablishment oscillator under the exclusive control of the lowsignal detection relays, is to prevent the oscillator from going intoeffect and knocking out the main carrier which may be so weak as toenergize the low signal Sensitrol detecting relays but neverthelesssuiiiciently strong to maintain communication. The Sensitrol relays areset to detect such a carrier since this condition must be made known tothe terminal operators while at the same time carrier reestablishment isnot necessary. As propagation and other conditions vary from day to day,only at the terminal is it possible to tell when the system has actuallyfailed. Hence, there is provided the dual mutually de pendent control ofthe carrier reestablishment oscillator energization circuit.

It should be noted that the above description of the repeater apparatuswas based upon West to east transmission. Duplicate apparatus isprovided at each repeater (as disclosed in Fig. 1) for east to westtransmission.

It is, of course, understood that the specific embodiment of theinvention herein disclosed is by way of example only and variousmodifications and changes may be made without departing from the scopeor spirit of the invention as defined in the appended claims.

What is claimed is:

1. In a radio relay system having a repeater station and two terminalstations, apparatus at said repeater station including a receiver and atransmitter, fault dctecting means connected to said receiver, carrierreestablishment generating means having an energizing circuit andanoutput circuit, said output circuit being coupled to said transmitter,two normally open switches in said energizing circuit, means undercontrol of said fault detecting means to close one of said normally openswitches, and means responsive to a signal from a terminal station toclose the other of said normally open switches to complete theenergization circuit of said carrier reestablishment means.

2. In a radio relay system having two terminal stations and at least onerepeater station, apparatus at the repeater station including a receiverand transmitter, fault detecting means connected to said receiverresponsive to received signals below a predetermined power level, acarrier reestablishment oscillator with an energizing circuit and anoutput circuti, means coupling the output circuit to said transmitter,means responsive to Said fault detecting means to prepare saidenergizing cir.- cuit, and means responsive to a signal from a terminalstation to complete said energizing circuit.

3. ln a radio relay system having two terminal stations and at least onerepeater station, apparatus at said rer, peater station including areceiver and a transmitter, fault detecting means coupled to saidreceiver and set to respond upon received signals `falling below apredetermined power level, a fault tone oscillator, means responsive tosaid fault detecting means to energize said fault tone oscillator,`means coupling the output ofsaid fault ltone oscillator to thetransmitter to modulate the carrier signal thereby indicating a faultedcondition, va carrier reestablishmeut means wtih an Venergizing circuitand an output circuit, means coupling `the output circuit of the carrierreestablishment means to the transmitter, other means responsive toVsaid fault detecting meansto prepare said energizing circuit, and meansresponsive to as'ignal from a terminal station to complete theenergizing circuit thereby reestablishing a faulted carrier to permitthe fault tone oscillator to signal the location of a faulted conditionto a terminal station.

4. In a radio relay system having two terminal stations and a pluralityof repeater stations, apparatus at each repeater station including areceiver and a transmitter, fault detecting means coupled to saidreceiver and set to respond when the main carrier signal is notreceived, a fault tone oscillator, the fault tone oscillator at eachrepeater being designed to oscillate at a different characteristicfrequency, means responsive to said fault detecting means to energizesaid fault tone oscillator, means coupling the output of said oscillatorto the transmitter, a carrier reestablishment means with an energizingcircuit and an output circuit, means coupling the output circuit of thecarrier reestablishment means to the transmitter, other means responsiveto said fault detecting means to prepare said energizing circuit, andmeans responsive to a signal from a terminal station to complete theenergizing circuit thereby reestablishing a faulted carrier to permitthe fault zone oscillator to signal the location of a faulted conditionto a terminal station. i 5. In a radio relay system including twoterminal stations and an intermediate repeater station, apparatus atsaid repeater station comprising a first receiver and ltransmitter forrelaying radio signals in one direction, a second receiver andtransmitter for relaying radio signals in another direction, faultdetecting means coupled to ysaid first receiver and to said secondreceiver, fault tone means,

means responsive to said fault detecting means to energize said 'faulttone means, means coupling the output of said fault tone means to saidfirst and second transmitters, 'a yfirst carrier reestablishmentoscillator with an energizing circuit and an output circuit, meanscoupling said output circuit to said first transmitter, a secondAcarrier reestab- F lishment oscillator with an energizing circuit andan output circuit, means coupling the output circuit of the sec ondcarrier reestablishment oscillator to said second transmitter, othermeans responsive to said fault detecting means to condition each of saidenergizing circuits, means responsive to a signal from one terminalstation to cornplete the energizing circuit ot the first carrierreestablishment `oscillator and means responsive to a signal from theother terminal station to complete the energizing circuit of the lsecondcarrierrreestablishment oscillator.

6. In aradio relay system including two terminalgstations and anintermediate repeater station, apparatus at said repeater stationcomprising a first receiver and a vtirst transmitter for relaying radiosignals in one direction, a second receiver and a lsecond transmitterfor relaying radio signals in another direction, a first fault detectingmeans coupled to said first receiver, second fault detecting meanscoupled tovsaidtsecond receiver, a fault tone oscillator, .meansresponsive tosaid 'trst'tault detecting means to' energize saidoscillator, means vresponsive to said `see ond fault detecting means toenergize said oscillator, means coupling the output of the fault toneoscillator to said first transmitter and to said second transmitter, afirst carrier reestablishment oscillator with an energizing circuit andan output circuit, means coupling the output circuit of the firstcarrier reestablishment oscillator to the first transmitter, a secondcarrier reestablishment oscillator with an energizing circuit and anoutput circuit, means coupling the output circuit of the second carrierreestablishment oscillator to the second transmitter, other meansresponsive to said first fault detecting means to condition theenergizing circuit of the first carrier reestablishment oscillator,other means responsive to said second fault detecting means to conditionthe energizing circuit of the second carrier reestablishment oscillator,means responsive to a signal from one terminal station to complete theenergizing circuit of the first carrier .reestablishment oscillator andmeans responsive to a signal from the other terminal station to completethe energizing circuit of said second carrier reestablishmentoscillator.

7. In a radio relay system including two terminal stations and aplurality of intermediate repeaters, each repeater comprising a firstreceiver and a first transmitter for relaying radio signals in onedirection, a second receiver and a second transmitter for relaying radiosignals in another direction, first fault detecting means connected tosaid first receiver, second fault detecting means connected to saidsecond receiver, each of said first and second fault detecting means setto respond when a main carrier signal is not received by the first andsecond receivers respectively, a fault tone oscillator, the fault toneoscillator at each repeater being designed to oscillate at a differentcharacteristic frequency, means responsive to said first fault detectingmeans to energize said oscillator, means responsive to said second faultdetecting means to energize said oscillator, means coupling the outputof the fault tone osciliator to said first transmitter and to saidsecond transmitter, a first carrier reestablishment oscillator with anenergizing circuit and an output circuit, means connecting the outputcircuit of the first carrier rcestablishment oscillator to the firsttransmittena second carrier reestablishment oscillator with anenergizing circuit and an output circuit, means connecting the outputcircuit of the second carrier reestablishment oscillator to thesecondtransmitter, other means responsive to said lirst fault-detecting meansto condition the energizing c ircuit -ofr the first carrierreestablishment oscillator, other means responsive to said second faultdetecting means to condition the energizing circuit of the secondcarrier reestablishment oscillator, means responsive to a signal fromone terminal station to complete the energizing circuit of the tirstcarrier reestablishment -oscillator ,thereby reestablishing a faultedcarrier in one direction to permit the fault tone oscillator to signalthe location of a faulted condition, and means responsive to a signalfrom the other terminal station to complete the energization circuitofthe second carrier reestablishrnent oscillator thereby reestablishing afaulted carrier in the other direction to permit the fault toneoscillator to signal the location of a faulted condition.

References Cited in the file of this patent UNITED STATES PATENTS 5w me

